Vapour sorption and solvatochromism in a metal-organic framework of

Article pubs.acs.org/crystal

Cite This: Cryst. Growth Des. 2018, 18, 416−423

Vapor Sorption and Solvatochromism in a Metal−Organic Framework of an Asymmetric Pyridylcarboxylate Christelle N. Dzesse T,†,‡ Emmanuel N. Nfor,‡ and Susan A. Bourne*,† †

Centre for Supramolecular Chemistry Research, Department of Chemistry, University of Cape Town, Rondebosch 7701, Cape Town, South Africa ‡ Department of Chemistry, University of Buea, Cameroon, P.O. Box 63, Buea, Cameroon S Supporting Information *

ABSTRACT: Two new metal−organic frameworks (MOFs) of cobalt(II) and 3-(4-pyridyl)benzoate (34pba) were prepared concomitantly under solvothermal conditions. Using dimethylformamide (DMF) as solvent, {[Co(34pba)2(H2O)]·1/2DMF·H2O}n, 1, can be prepared as a pure phase in the presence of the templating molecules arginine or proline, neither of which is incorporated in the resulting structure. {[Co(34pba)2]·2DMA}n, 2, (where DMA is dimethylacetamide) has the same framework formula as two previously reported MOFs and represents a third in this series of structural isomers. The activated phase of 1, 1d absorbs a number of solvent vapors, across a wide range of polarities, with accompanying phase changes and solvatochromic responses. The facile exchange of guest species within the framework of 1d is explained by the low activation energy (59−69 kJ mol−1) for the desorption of water from 1d·H2O.

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INTRODUCTION Metal−organic frameworks (MOFs) are crystalline materials made by combining metal ions or clusters with polytopic organic linkers. The resulting networks have variable dimensionalities and porosity.1−3 MOFs with flexible structures which react to stimuli, often by transforming between distinct crystalline phases, are of particular interest as sensors. Flexibility can be built in to the MOF by using appropriate linkers and metal ions with variable coordination geometries.4 The more flexible the linker, the more likely it is that one obtains interpenetrating MOFs, resulting in reduced porosity. Thus, a compromise is required, in order to balance the competition between porosity and stimuli response. One form of sensing behavior is observed when a material visibly changes color on sorption of solvent vapor, a phenomenon known as solvatochromism. A recent review of chromic effects in MOFs highlighted solvatochromism as an area ripe for exploitation in developing useful devices.5 In general, solvatochromism arises from changes in the metal coordination geometry or from modifications to the framework structure which affect the metal to ligand charge transfer (MLCT), d−d, or π−π* transitions.6,7 Solvatochromism is frequently facilitated by guest exchange in the MOF voids. While many MOFs are unstable in highly aqueous media, a recent study reported a series of MOFs which regenerate after dissolution in water.8 Using a salt metathesis process, the authors could regenerate the same frameworks after dissolution, resulting in cationic frameworks with interchanged anions contained in the voids. Even when starting with the same reactants, the MOFs obtained are frequently not predictable, as synthetic conditions © 2017 American Chemical Society

play a crucial role in templating the framework formed. Thus, a number of authors have investigated factors such as temperature, pH, solvent, and time of reaction.9−13 Furthermore, substitution of hydrogen atoms on the organic linker by fluorination or perfluorination has been shown to influence the properties of the MOFs for applications such as gas sorption.14−16 Supramolecular isomerism is the term used to describe MOFs which have the same chemical composition in their networks, while the solvent or guest content of the networks need not be the same.17−20 We recently reported two complexes of {[Co(34pba)2]·DMF}n (where 34pba is 3-(4-pyridyl)benzoate), which, although they have the same empirical formula, are not supramolecular isomers as their coordination entities differ. Instead these were described as molecular building block (MBB) structural isomers.21 Here we will refer to these as M1 and M2. In this work we report the preparation of two further MOFs formed from the same components, yielding {[Co(34pba)2(H2O)]·1/2DMF· H2O}n (1) and {[Co(34pba)2]·2DMA}n (2). These complexes are formed concomitantly, but 1 can be isolated alone when a templating molecule is used in the reaction mixture, although this is not included in the resulting structure. The solvatochromic response of 1 to a range of polar and apolar solvents was tested, and the kinetics of desorption of water from its inclusion compound with the activated form of 1 is also reported. Received: October 6, 2017 Revised: November 13, 2017 Published: November 16, 2017 416

DOI: 10.1021/acs.cgd.7b01417 Cryst. Growth Des. 2018, 18, 416−423

Crystal Growth & Design

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Article

(0.4 mmol, 46 mg); however, these amino acids are not incorporated into the compound obtained. Single Crystal Structure Refinement. Data collections were performed on a Bruker KAPPA Apex II Duo diffractometer using MoKα (λ = 0.71069 Å). X-rays generated by a Bruker K780 generator powered at 50 kV and 30 mA. Single crystals of good quality were selected from mother liquor under the microscope and covered immediately in paratone N oil22 to prevent solvent loss or crystal decomposition. They were maintained at 173(2) K using a Cryostream cooler. Unit cell refinement and data reduction were performed using the program SAINT.23 Data were corrected for Lorentz polarization effects and for absorption using the program SADABS24 and refined by full-matrix least-squares on F2 with anisotropic thermal parameters for all nonhydrogen atoms using SHELXL25 in the X-SEED26 interface. All hydrogen atoms were placed in calculated positions and refined isotropically. Crystallographic data of compounds 1 and 2 are presented in Table 1, while some hydrogen bonds and selected bond lengths and angles for 1 and 2 can be found in the Supporting Information. Analytical Methods. Elemental analysis was performed on a Fisons EA1108 CHNS-O elemental analyzer. IR spectra were recorded on a PerkinElmer Spectrum Two Fourier transform infrared (FT-IR) fitted with a UATR diamond reflectance device for solid samples. Elemental Analyses. For 1, Found: C, 58.02, H, 4.48, N, 6.23%, Calculated for CoC25.5H23.5N2.5O6.5: C, 58.02, H, 4.49, N, 6.63% For 2, Found: C: 60.88, H: 5.21, N: 8.48%. Calculated for CoC32H34O6N4 C: 61.05, H: 5.44, N: 8.90%. IR (cm−1): (1): 3274 (br), 1611(m), 1548(m), 1505(s), 1391(s), 1223(s), 1016(s), 1016 (s), 743(s), 676(s) (2): 3450 (br), 2960(br), 1611(m), 1538(s), 1504(s), 1441(s), 1392(s), 1266(s), 742(s), 681(s) Thermal Analysis. The thermal behavior of 1 and 2 was monitored using a Nikon SMZ-10 stereoscopic microscope fitted with a Linkam THMS600 hot stage and a Linkam TP92 temperature control unit. The samples were placed on a coverslip under silicone oil and heated at 10 °C

EXPERIMENTAL SECTION

Materials. Cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O) (98% purity), L-proline (Pro), and L-arginine (Arg) (purity greater than 99%) were purchased from Sigma-Aldrich. 3-(Pyridine-4-yl)benzoic acid (34pbaH) (95% purity) was purchased from CGene Tech Inc. All materials were used without any further purification. Syntheses of {[Co(34pba)2(H2O)]·1/2 DMF·H2O}n (1) and {[Co(34pba)2]·2DMA}n (2). In the initial reaction, 1 and 2 crystallized concomitantly in a one-pot solvothermal synthesis: Co(NO3)2·6H2O (0.1 mmol, 29 mg) was dissolved in water, 34pbaH (0.4 mmol, 80 mg) was dissolved in dimethylformamide (DMF), and the two solutions were mixed in a vial, sealed tightly, and kept in an oven at 105 °C for 72 h, followed by a fast cooling to room temperature. Red purple prisms of 1 and dark purple blocks of 2 were obtained (Figure 1). On structure

Figure 1. Crystals of 1 (left) and 2 (right). solution, the DMF in 2 was found to have reacted with itself to form the methylated derivative dimethylacetamide (DMA). Altering the temperature of the oven did not allow separate syntheses of the pure forms of 1 and 2. 2 was always present in smaller quantities and had to be handseparated from 1 for further analysis. 1 could be obtained as a pure phase in the presence of the amino acids arginine (0.4 mmol, 70 mg) or proline

Table 1. Crystal Data and Refinement Parameters of 1, 2, and Related Published Structures

a

compound

1

2

M1a

M2a

molecular formula molecular mass (g mol−1) crystal size (mm) temperature (K) crystal symmetry space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z D (g·cm−3) μ (Mo−Kα) (mm−1) F(000) range scanned, θ (deg) index ranges (h, k, l) no. reflections collected no. unique reflections no. reflections with I ≥ 2σ(I) parameters/restraints goodness of fit, S final R indices (I ≥ 2σ(I)) final wR2 (all data) min, max e density (e Å3)

C25.5H23.5CoN2.5O6.5 527.90 0.13 × 0.15 × 0.17 173(2) triclinic P1̅ 7.9001(5) 11.6244(7) 14.6627(9) 71.255(1) 83.456(1) 70.522(1) 1202.10(13) 2 1.458 0.762 546 1.47; 28.39 −10:10; −15:15; −19; 19 30275 6031 5192 326/2 1.072 0.0570 0.1408 −0.50, 2.48

C32H34CoN4O6 629.56 0.17 × 0.19 × 0.25 173(2) orthorhombic C2221 15.3830(8) 17.2032(9) 11.6898(6) 90 90 90 3093.6(3) 4 1.352 0.604 1316 1.77; 28.37 −20:20; −20:22; −15:15 22222 3871 3633 246/0 1.051 0.0349 0.0879 −0.47, 0.48

C27H23CoN3O5 528.41

C27H23CoN3O5 528.41

173 orthorhombic Pbca 18.0941(7) 14.3902(6) 19.1497(8) 90 90 90 4986.2(4) 8 1.408 0.731

173 tetragonal P43212 11.4957(8) 11.4957(8) 37.257(4) 90 90 90 4923.6(3) 8 1.423 0.740

Reported in ref 21. 417



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Figure 2. (a) Coordination environment of the cobalt(II) ion in 1 (hydrogen atoms omitted for clarity), (b) packing in 1 down [100], (c) packing in 1 down [001], with guest DMF and water molecules shown as spheres and framework as sticks. DMF and water molecules occupy separate channels. min−1. Thermal events were monitored using a Sony Digital Hyper HAD color video, and the images were captured at different temperatures using the Soft Imaging System Program.27 Thermogravimetric analyses (TGA) were performed using a TA Instruments, TA-Q500 under dry nitrogen gas at a flow rate of 60 mL min−1. Samples with a mass in the range 1−5 mg were placed in an open platinium crucible and heated at 10 °C min−1. Differential scanning calorimetry (DSC) analyses were recorded on a TA Instruments, DSC-Q200, under nitrogen purge gas flowing at a rate of 60 mL min−1, or on a DSC XP-10 TH1SS with nitrogen gas at a flow rate of 40 mL min−1. Samples with masses 1−2 mg were placed in vented aluminum pans and heated at 10 °C min−1. In all cases, samples were dried on filter paper prior to analysis. TGA and DSC traces are provided in the Supporting Information. As only a few dark purple crystals could be isolated from the mixture, no further characterization or applications for 2 could be studied. Powder X-ray Diffraction (PXRD). PXRD was used to confirm that the single crystal structure was representative of the bulk material and to show that samples of 1 exposed to various solvents have different phases from that of the original material. Samples were analyzed using a Bruker D8 advanced diffractometer equipped with a Lynxeye detector using CuKα radiation (λ = 1.5406 Å) at 298 K. Crystals removed from mother liquor were dried on filter paper before being crushed into a fine powder using a mortar and pestle and then placed on a zero-background sample holder. X-rays were generated by a current flow of 40 mA and an accelerating voltage of 30 kV. Variable temperature PXRD (VTPXRD) was performed over a temperature range of 25−300 °C, X-rays being generated by a current flow of 40 mA and a voltage of 40 kV. All the samples were scanned within the range 4−40° with a step size of 0.020° to give a total of 1760 steps. Preparation of {[Co(34pba)2]}n (1d). Crystals of 1 were dried under a vacuum for 10 h at 180 °C. A purple powder (1d) was obtained. Despite repeated efforts using a variety of experimental conditions, it proved impossible to obtain single crystals of 1d. PXRD analysis shows that the crystallinity decreases and the phase of 1d is notably different from the solvated form 1. TGA analysis of 1d showed no mass loss before decomposition above 400 °C (Figure S6). Elemental analysis results of 1d found C 62.12, H 3.52, N 5.84%. Calculated for CoC24H16N2O4: C 63.31, H 3.54, N 6.15%.

IR (cm−1): 3669 (br), 2969 (br), 1612 (m), 1571 (m), 1543(s), 1221 (s), 1068 (s), 878 (s), 743 (s). Vapor Sorption and Kinetics Studies on 1d. The synthesized compound 1 was activated under a vacuum at 180 °C for 10 h to yield 1d. A color change was apparent between the initial and the dried sample. TGA was run on the dried sample to ensure it was totally free of pore solvents before exposure to different solvents including dichloromethane (CH2Cl2), chloroform (CHCl3), 1,1,1-trichloroethane, trichloroethylene, chlorobenzene, methanol (MeOH), ethanol (EtOH), water (H2O), 1,4-dioxane, acetone, dimethylformamide (DMF), dimethylacetamide (DMA), dimethyl sulfoxide (DMSO) as well as several binary mixtures of these solvents. To study the uptake of solvent by 1d, samples were placed in small vials without lids. The small vials were then placed in large ones containing the respective dry solvents. These large vials were capped, sealed, and left at room temperature for several hours to days. Most samples changed color during this time, details provided in the Results. The inclusion compounds obtained were analyzed using TGA, hot stage microscopy (HSM), PXRD, elemental analysis, and FT-IR. Analyses are detailed below or in the Supporting Information. 1d-water was selected to study the non-isothermal kinetics of desorption. TGA was run at different heating rates (2, 4, 8, 16, and 32 °C min−1). Data obtained were analyzed using the Universal Analysis 2000 program28 and Microsoft Excel. The sample size used was relatively constant (1.0−3.2 mg) for each experiment. The material spread at the bottom of the pan was analyzed using different heating rates at a given temperature. The activation energy for guest desorption was calculated using the Ozawa, Flynn, and Wall method.29 This method consists of using several heating rates and temperatures that are recorded at a specific conversion level for each heating rate using the following equation:

⎛E ⎞ ⎛ AE ⎞ log βi = log⎜ i ai ⎟ − 2.315 − 0.457⎜ ai ⎟ ⎝ f (i)R ⎠ ⎝ RTi ⎠

(1)

where βi is the heating rate, Ai the frequency factor, Eai the activation energy, Ti is the temperature at each conversion level and f(i) the kinetic model applied. 418



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Figure 3. (a) Coordination environment of Co(II) in 2; (b) packing of 2 viewed along [001] showing guest molecules (shown with van der Waals radii) in channels.

Figure 4. Photographs of a crystal of 2 on the hot-stage microscope, illustrating color changes with increasing temperature.

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RESULTS AND DISCUSSION Crystal structures of {[Co(34pba)2(H2O)]·1/2 DMF· H2O}n (1) and {[Co(34pba)2]·2DMA}n (2). 1 and 2 were crystallized concomitantly in a solvothermal synthesis at 105 °C. Despite repeated crystallizations under different conditions of temperature and solvent, crystals of 2 were never isolated alone, but could be hand selected and separated under a microscope because of their distinct morphology and color. 1 could be selectively prepared and crystallized, but only in the presence of the amino acids proline or arginine. While the presence of spectator ions is well recognized as influencing the products of crystallization, there are fewer reports of the influence of neutral spectator species. The effect is known however. For example, Halper and Cohen used a fluoro-derivative to influence the polymeric coordination complex obtained,30 while Zhang et al. found that chiral spectator solutes induced homochiral framework crystallization.31 The crystal data and structure refinement parameters of 1, 2, and two related structures previously reported are listed in Table 1. The crystal structure of 1 contains one independent cobalt(II) ion that is octahedrally coordinated, bound to four 34pba linkers and one water molecule via two Co−N and four Co−O bonds as shown in Figure 2. One 34pba exhibits a chelating coordination mode through the carboxylate moiety, while the other binds in a monodentate fashion. The coordination environment around each cobalt atom is similar. Co−N bond lengths are 2.104 and 2.140 Å, while Co−O is between 2.034 and 2.214 Å which are within expected ranges. Selected bond lengths and angles are listed in Tables S1 and S2 respectively. The octahedron is slightly distorted due to the bidentate binding mode of one carboxylate moiety. When crystallized, there is a disordered DMF molecule (sof 0.5) and a water molecule included in the cavity space. The packing of 1 viewed along [100] and [001] is also displayed in Figure 2. The packing along [001] shows interdigited layers with

water and disordered DMF molecules in the cavities, with the pyridyl moieties from two cobalt facing one another. Several π−π interactions are observed in the network as well as O−H···O and C−H···O hydrogen bonds (Table S3). PXRD showed a good match between the experimental trace and that calculated for 1, indicating that phase purity could be obtained (Figure S1). All the guest molecules as well as the coordinated water were removed in a single step on heating as observed on the TGA trace (Figure S2) which shows a total mass loss of 13.7% (calculated 13.8%) between 40 and 175 °C. 2 crystallizes in the orthorhombic system, space group C2221. Crystal data and structure refinement can be found in Table 1. Its asymmetric unit consists of a cobalt(II) ion with site occupancy of 50%, one 34pba ligand, and two uncoordinated dimethylacetamide (DMA) guest molecules. The cobalt ion is hexacoordinated, coordinated to two carboxylate groups in the bidentate mode and two pyridyl nitrogens, hence forming an octahedron which is distorted (Figure 3). As in 1, the Co−N (2.090 Å) and Co−O (2.076 and 2.228 Å) bonds are in the normal range for Co(II) complexes. Selected bond lengths and angles can be viewed in Tables S4 and S5 respectively. The DMA molecules are disordered over two positions, each of them having a site occupancy factor of 0.5. 2 is a neutral 2D-network with channels running parallel to the c-axis filled with DMA molecules. Its packing diagram along [001] is displayed in Figure 3. As in 1, weak intermolecular C− H···O hydrogen bonds exist in the network (table S6). PXRD of crystals hand-selected and crushed showed a good match between the experimental trace and that calculated for 2 (Figure S3). The desorption of DMA occurs over a range of 100−240 °C, with an observed mass loss of 27.1%, which corresponds well to the calculated mass loss of 27.7% (Figure S4). The DSC for 2 shows an exothermic peak at 265 °C, which corresponds to a phase transition observable on the HSM. Figure 4 presents 419



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Figure 5. Powdered 1d (center) and after exposure to solvents: (a) dichloromethane, (b) chloroform, (c) 1,1,1-trichloroethane, (d) trichloroethylene, (e) chlorobenzene, (f) methanol, (g) ethanol, (h) water, (i) 1,4-dioxane, (j) acetone, (k) DMF, (l) DMA, (m) DMSO.

Table 2. Thermal Analysis Results of Solvent−Sorption Experiments Using a Single Solvent inclusion into 1d:solvent dichloromethane chloroform 1,1,1trichloroethane trichloroethylene chlorobenzene methanol ethanol water 1,4-dioxane acetone DMF DMA DMSO

mass loss %

temperature range of mass loss (°C)

3.0 9.2 5.2

79−95 99−120 86−106

2.0 1.0 6.6 15.6 13.8 47.1 12.9 56.5 50.8 55.5

71−128 56−94 25−113 25−94 99−118 25−92 101−126 25−113 25−121 25−118

color

solvent boiling point (°C)

dipole moment (D)

40 61 74

1.60 1.04 1.90

negligible 1:0.39 1:0.19

87 131 65 78 100 101 56 153 165 189

0.81 1.70 1.70 1.69 1.85 0.45 2.88 3.82 3.72 3.96

negligible negligible 1:1.0 1:1.8 1:4.0 1:4.6 1:1.2 1:8.1 1:5.4 1:7.3

violet violet violet violet violet pink pink yellow pale pink pale pink orange pink orange

H-bond donor/ acceptor

HBD/HBA HBD/HBA HBD/HBA HBA HBA HBA HBA HBA

H:G ratio (based on 1d)

coordinated to cobalt(II). XEVXUT and XEVXON have frameworks with formula [Co(34pba)2]n′, analogous to compound 2. XEVXUT was reported as having the topology of a 3D bcu net, while XEVXON is a 2D sql net.21 Although 2 crystallizes in a different space group with a very different unit cell (a consequence of the different guest species and a H:G ratio of 1:2 rather than 1:1 as in XEVXON) it too displays the network connectivity of a two-dimensional square grid with topology symbol sql. FUMJAZ01 is a MOF with the same formula as compound 1, [Co(34pba)2(H2O)]n. Their unit cells are significantly different (1 crystallizes in P1̅ and FUMJAZ01 in C2/c), and 1 has a ladder topology while FUMJAZ01 is a 3D cds network.33 The remaining Co(II) MOF with 34pba is KIRROT with framework [Co2(34pba)3(μ-OH)], which forms a chiral helical double-layered structure.34 One further structure in the CSD is

photographs of a crystal of 2 at different temperatures. The crystal color lightens from dark to light purple between room temperature and 210 °C. By 290 °C the crystal is violet and changes to blue by 315 °C. The latter change can be attributed to a change in the coordination sphere around the cobalt(II) ion which becomes tetrahedrally coordinated as the 34pba linkers change to monodentate coordination.32 This effect was previously observed in related compounds M1 and M2.21 The difference in temperature in the HSM and DSC is probably due to the different environments (open/closed) in which they are carried out. Comparison of 1 and 2 with Related Structures in the Literature. The Cambridge Structural Database (Version 5.38, May 2017) contains 38 MOFs in which 34pba is coordinated to transition metals, and a further nine in which it is coordinated to a lanthanide ion. Of the former, five structures have 34pba 420



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Figure 6. Representative PXRD patterns of 1d-solvent for several classes of inclusion. From the bottom: 1, 1d, 1d-dichloromethane (cf. inclusion with trichloroethane, trichloroethylene, chlorobenzene), 1d-chloroform, 1d-ethanol (cf. inclusion with acetone, dioxane, water), and 1d-DMSO (cf. inclusion with DMF and DMA).

IDECOL [Co(34pba)2(H2O)4)]n, which is a 3D hydrogenbonded coordination complex.35 Sorption Studies on the Activated Phase of 1. Variable temperature PXRD (Figure S5) confirmed that 1 changes phase to 1d at a temperature of ca. 100 °C; this phase is retained until crystallinity is lost at ca. 200 °C. To obtain the activated phase, 1d, powdered crystals of 1 were dried under a vacuum at 180 °C for 10 h. The crystals changed color from red purple to violet, and the TG of the evacuated sample (Figure S6) showed that no solvent was retained. 1d was then exposed to the vapors of various solvents as described in the Experimental Section. Uptake of vapor was indicated by the change of color observed, a process known as solvatochromism, as illustrated in Figure 5, and confirmed by TGA and DSC (Table 2). TG and DSC traces of solvated phases can be found along with PXRD traces (Figures S7 and S8) in the Supporting Information. The DSC profiles show a broad endothermic peak in each case, corresponding to the desolvation process. In some cases, a small exothermic peak at ca. 270 °C indicates a phase transition prior to the decomposition of the framework. This phase transition is the irreversible change in coordination about the cobalt ion, as the carboxylate changes from bidentate to monodentate coordination.36 The relative stabilities of the solvated forms are provided in Table 2. Figure 6 shows the PXRD traces of representative solvated forms, some of which remain similar to the phase of 1d, while others show distinct differences. DMF, DMA, and DMSO are all similar to one another and different to 1d. Acetone, dioxane, ethanol, and water form another group with similar PXRD traces, and a further group can be distinguished in the chlorinated solvents dichloromethane, trichloroethylene, and trichloroethane. The latter group retains the closest similarity to the pattern of 1d, which is unsurprising as these solvents are taken up in very small quantities. Interestingly, the pattern for 1dchloroform is a good match to that of the group represented by 1d-ethanol. We note that these groups of related phases correspond to the colors of the respective powders (Table 2). There appears to be a preference for guest species that are

hydrogen-bond acceptors, with the more polar of these having higher H:G ratios. As observed previously for a similar system, the powder changes color most notably on sorption of solvents capable of participating in hydrogen-bonding,36 and result in a change in the crystalline phase (Figure 6). The color changes are derived from the flexibility of the framework induced by solvent hydrogen bonding, giving rise to changes in the d−d transitions in the visible region. Similar effects have been reported previously.37−39 However, in this case, we note that the frameworks of the solvated forms show some differences from one another (in three groups), suggesting that it is possible that more direct d−d transitions are responsible for the colors observed. In addition to the sorption of individual solvents, samples of 1d were exposed to equimolar mixtures of two solvents. In most cases, one of the solvents was a polar aprotic hydrogen-bond accepting molecule (DMF, DMA, DMSO), while the other was one of water, methanol, ethanol, or acetone. PXRD traces and TG/DSC are shown in the Supporting Information and results are summarized in Table 3. Where water was available, this was preferentially taken up to the same H:G ratio as observed for water sorption alone. Where water was not one of the solvents available, the polar aprotic solvent was selected. This is an Table 3. Thermal Analysis Results of Solvent−Sorption Experiments Using Mixed Solvents

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Inclusion into 1d: mixed solvents

mass loss %

temperature range of mass loss (°C)

solvent included

H:G ratio (based on 1d)

DMF/H2O DMA/H2O DMSO/H2O DMF/MeOH DMA/EtOH DMSO/EtOH DMF/acetone DMSO/acetone

14.2 13.6 14.3 44.0 44.3 43.9 52.4 41.1

107−136 99−121 105−128 83−109 97−125 85−133 79−121 8−121

H2O H2O H2O DMF DMA DMSO DMF DMSO

1:4.2 1:4.0 1:4.2 1:4.9 1:4.2 1:4.6 1:6.9 1:4.1



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Table 4. Reciprocal Temperature Values Obtained at Various Conversion Levels and for Each Heating Rate 1000/T(K) for % conversion heating rate (β)/K·min−1

log β

7%

22%

36%

51%

80%

2 4 8 16 32

0.301 0.602 0.903 1.204 1.505

2.898 2.808 2.728 2.649 2.534

2.791 2.707 2.652 2.561 2.474

2.742 2.666 2.606 2.519 2.434

2.717 2.635 2.577 2.459 2.409

2.672 2.593 2.534 2.442 2.365

the desorption of water from the resulting compound is 59−69 kJ mol−1, indicating the ease with which water (and other solvents) can be sorbed and desorbed from this material.

interesting result as these were the solvents with lower volatility, thus indicating a definite interaction preference rather than one based simply on vapor availability. Kinetics of Desorption of Guest Water from a Solvated Phase of 1. On uptake of solvents, the crystallinity of 1d reduced so that no single crystal data collection could be carried out on the solvated forms. To determine more information about the nature of the interactions between solvent and the framework, we investigated the kinetics of desorption of water from 1d-H2O. In this experiment we used non-isothermal thermogravimetry to determine the activation energy associated with the dehydration of 1d-H2O by the Ozawa, Flynn, and Wall method described in the Experimental Section.29 TG traces were measured at 2, 4, 8, 16, and 32 K min−1 (Figure S10). The percentage conversion level at a given temperature was determined for each trace (Table 4). A graph of log β vs 1000/ T (Figure 7) shows linear curves which are almost parallel

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01417. Crystallographic data, thermal analysis, and PXRD data (PDF) Accession Codes

CCDC 1578519−1578520 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Susan A. Bourne: 0000-0002-2491-2843 Funding

C.D.T. thanks the Organization for Women in Science for the Developing World (OWSD) for financial support and the University of Cape Town for hosting her for part of the duration of her studies. This project was supported in part by funding from the South African National Research Foundation (Grant 90495). Notes

The authors declare no competing financial interest.

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Figure 7. Plot of log β versus 1000/T(K) for 1d-water dehydration at various levels of conversion.

REFERENCES

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(indicating the same mechanism for dehydration at each level of conversion). By equating the gradient of the curves to −0.457 Ea/ R (eq 1), activation energies were calculated as falling in the range 59.1−68.6 kJ mol−1 (details in Table S7). These values are similar to others previously reported for desolvation processes.40,41

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CONCLUSION Two new metal−organic frameworks of Co(II) and 34pba have been synthesized and characterized. Although 2 was only formed concomitantly with 1, we were able to prepare samples of pure 1 by using neutral spectator molecules proline or arginine in the reaction mixture. After activation, 1d was capable of vapor sorption of a range of solvents, with an accompanying solvatochromic response. There was some selectivity observed when 1d was exposed to a binary mixture of vapors, with a distinct preference for water sorption. The activation energy for 422



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Vapour sorption and solvatochromism in a metal-organic framework of

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